The present invention relates to an image forming apparatus which forms a toner image by using a liquid developer in which toner is dispersed in solvent.
In the image forming apparatus, for example, a copying apparatus in which an electrostatic copying process is utilized, an electrostatic latent image corresponding to an image of an original which is a copy object transferred on a photosensitive body as a clear contrast between light and shade is formed and copied by being visualized by toner which is a developer. As a method for supplying toner to an electrostatic latent image, there is known the liquid developer in which toner is dispersed in a solvent.
The image forming apparatus using the liquid developer can use toner having a small particle size in comparison to the image forming apparatus of a magnetic brush developing and is useful in order to enhance (high quality imaging) image quality.
The developer is mainly constituted by toner, a charge control agent and a solvent, and a counter ion having a polarity in reverse to the toner charged to a predetermined polarity, an excessive-ion (of the charge control agent) and the like suspend in the developer so as to bear an electric conductivity.
With respect to an image portion in which the electrostatic latent image exists in the developing area where, for example, the photosensitive body and the developer bearing body are located at a predetermined interval, the toner in the developer migrates toward the electrostatic latent image by the potential difference defined by the electrostatic latent image (the area in which the potential on the photosensitive body is changed) and the developing bias voltage applied to the developer bearing body. On the other hand, the counter ion migrates toward the developer bearing body.
In contrast, with respect to a non-image portion where the electrostatic latent image on the photosensitive body does not exist in the developing area, the toner migrates toward the developer bearing body by the potential difference in a reverse direction to the image portion defined by the potential of the photosensitive body, which maintains a charged potential as it is charged, and the developing bias voltage applied to the developer bearing body. In this case, the counter ion migrates toward the photosensitive body.
Meanwhile, when the surface of the developer bearing body is metal, the counter ion is neutralized (at the developer bearing body side) since a charge injecting reaction occurs. However, since the surface of the photosensitive body is dielectric, the charge injecting reaction does not occur.
For this reason, in the image portion, the excessive-ion (the counter ion) generated by the attachment of the toner to the photosensitive body and the removal of the potential from the toner, is vanished by the charge injecting reaction at the developer bearing body side, while, in the non-image portion, the ion does not increase or decrease and the amount of excessive-ion does not fluctuate.
However, in order to correct for the decrease of the toner density inside the developer (liquid) by the repetition of the image forming, when a condensed toner liquid in which the toner is condensed is replenished in the developing liquid, the amount of excessive-ion inside the developer increases as the condensed liquid is supplied due to the influence of the excessive-ion contained in the toner condensed liquid.
When the amount of excessive-ion in the developing liquid increases, the conductivity of the developing liquid becomes high and there arise problems that the electrostatic latent image on the photosensitive body is vanished or a toner image is confused, wherein the toner electrically adhered on the electrostatic latent image flows into the non-image portion before it is fixed.
Further, when the amount of charge control agent to be added is changed in order to control the amount of excessive-ion inside the developing liquid, there arises a problem that not only the amount of excessive-ion, but also a charge applying capacity to the toner, which is an intrinsic role of the charge control agent, changes.
For this reason, for example, in Jpn. Pat. Appln. KOKOKU Publication No. 8-30921, it is disclosed that the amount of charge control agent in the condensed toner is decreased than the amount necessary and an insufficient portion is independently added after an elapse of a predetermined period of time. In this Publication, there is noted that, when adding the want charge control agent, the amount of the charge control agent to be added is controlled while measuring the conductivity.
However, in order that the charge control agent is dissociated and is adhered on the charging site of each toner particle so that the charging amount of toner is stabilized, it takes a time unit of several hours to several days and therefore that is a problem that the method for adding the charge control agent while measuring the conductivity is substantially not suitable for use in the image forming apparatus.
An object of this invention is to provide an image forming apparatus capable of controlling deterioration of a quality of an image caused by using a liquid developer in the image forming apparatus which uses the liquid developer.
This invention has been carried out in view of the above described problems and it is an object of this invention to provide an electrophotographic developing apparatus for developing an latent image on an photosensitive body by using a toner liquid where the toner is dispersed in non-polarity solvent and forming a toner image on the photosensitive body comprising:
a charging apparatus which applies a predetermined potential on the photosensitive body;
an exposing apparatus which forms an electrostatic latent image on the photosensitive body;
a developing apparatus which includes a developing roller opposed to the photosensitive body at a predetermined interval, supplies the toner liquid to the electrostatic latent image formed on the photosensitive body while maintaining the toner liquid between itself and the photosensitive body, and forms a toner image by selectively adhering the toner on the electrostatic latent image;
a toner liquid circulation mechanism which circulates the toner liquid maintained between itself and the photosensitive body by the developing roller of the developing apparatus while maintaining a toner density at a predetermined density;
a developing bias power supply capable of applying a predetermined bias voltage to the developing apparatus;
a conductivity detection mechanism which detects a conductivity inside the toner liquid circulated between the developing apparatus and the toner liquid circulation mechanism; and
a conductivity control apparatus which reduces the conductivity of the toner liquid circulated between the developing apparatus and the toner liquid circulation mechanism to a predetermined level or less by vanishing a surplus counter ion inside the toner liquid and a charge of an excessive-ion having the same polarity as the toner.
Further, an object of this invention is to provide a method for developing a latent image on a photosensitive body by using a toner liquid in which the toner is dispersed in a non-polarity solvent and forming a toner image on the photosensitive body comprising:
providing a predetermined potential to the photosensitive body;
forming an electrostatic latent image on the photosensitive body by exposing an image on the photosensitive body by an exposing apparatus;
selectively supplying and developing the toner from the toner liquid on the electrostatic latent image; and
replenishing an amount of toner consumed while developing the toner from the toner liquid on the electrostatic latent image by a condensed toner liquid in which a solid content density is higher than a density of the toner from the toner liquid, and reducing a conductivity of the toner liquid increased while replenishing the toner by a conductivity control apparatus so that an image forming condition is maintained constant.
Further, an object of this invention is to provide a developing apparatus for selectively supplying and developing a toner liquid in which toner particles are dispersed in a non-polarity solvent, on a latent image formed on a photosensitive body by using a toner liquid conveying mechanism and forming a toner image on the photosensitive body comprising:
a conductivity monitor apparatus which monitors the conductivity of the toner liquid; and
an apparatus which reduces the conductivity when the conductivity of the toner liquid monitored by the conductivity monitor apparatus exceeds a predetermined level.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.